Spinal cord stimulation (SCS) is a well-accepted clinical method for reducing pain in certain populations of patients. During SCS, the spinal cord, spinal nerve roots, or other nerve bundles are electrically stimulated using one or more neurostimulation leads implanted adjacent the spinal cord. While the pain-reducing effect of SCS is not well understood, it has been observed that the application of electrical energy to particular regions of the spinal cord induces paresthesia (i.e., a subjective sensation of numbness or tingling) that replaces the pain signals sensed by the patient in the afflicted body regions associated with the stimulated spinal regions. Thus, the paresthesia appears to mask the transmission of chronic pain sensations from the afflicted body regions to the brain.
The working clinical paradigm is that achievement of an effective result from SCS depends on the neurostimulation lead or leads being placed in a location (both longitudinal and lateral) relative to the spinal tissue such that the electrical stimulation will induce paresthesia located in approximately the same place in the patient's body as the pain (i.e., the target of treatment). If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy.
In a typical procedure, one or more stimulation leads are introduced through the patient's back into the epidural space under fluoroscopy. Stimulation energy may be delivered to the electrodes of the leads during and after the placement process in order to verify that the leads are stimulating the target neural tissue. Stimulation energy is also delivered to the electrodes at this time to formulate the most effective set of stimulus parameters, which include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation pulses at any given time, as well as the magnitude and duration of the stimulation pulses. During the foregoing procedure, an external trial neurostimulator may be used to convey the stimulation pulses to the lead(s), while the patient provides verbal feedback regarding the presence of paresthesia over the pain area. The stimulus parameter set will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit (e.g., pain relief), yet minimizes the volume of non-target tissue that is stimulated. Thus, neurostimulation leads are typically implanted with the understanding that the stimulus parameter set will require fewer than all of the electrodes on the leads to achieve the desired paresthesia.
After the lead(s) are placed at the target area of the spinal cord, the lead(s) are anchored in place, and the proximal ends of the lead(s), or alternatively lead extensions, are passed through a tunnel leading to a subcutaneous pocket (typically made in the patient's abdominal area) where a neurostimulator is implanted. The lead(s) are connected to the neurostimulator, which is programmed with the stimulation parameter set(s) previously determined during the initial placement of the lead(s). The neurostimulator may be operated to test the effect of stimulation and, if necessary, adjust the programmed set(s) of stimulation parameters for optimal pain relief based on verbal feedback from the patient. Based on this feedback, the lead position(s) may also be adjusted and re-anchored if necessary. Any incisions are then closed to fully implant the system.
A wide variety of neurostimulation leads have been introduced. One common type of neurostimulation lead is the percutaneous lead, which includes a plurality of spaced electrodes on a small diameter lead body. Percutaneous leads are relatively easy to place because they can be inserted into the epidural space adjacent the spinal cord through a percutaneous needle in a small locally-anesthetized incision while the patient is awake and able to provide feedback. One of the disadvantages of percutaneous leads, however, is that they are prone to migrating in the epidural space, either over time or as a result of sudden flexion movement.
Lead migration may relocate the paresthesia away from the pain site, resulting in the target neural tissue no longer being appropriately stimulated and the patient no longer realizing the full intended therapeutic benefit. With electrode programmability, the stimulation area can often be moved back to the effective pain site without having to reoperate on the patient in order to reposition the lead. Lead migration is, however, not the only reason that the therapeutic effects of a previously effective neurostimulation regimen will diminish or simply disappear, which can make diagnosis difficult. Moreover, even after a physician has determined that lead migration has occurred and that the system must be reprogrammed to accommodate the new positions of the electrodes, conventional neurostimulation systems do not provide the physician with information about the movement of an individual lead, such as how far one lead has moved relative to another lead. Knowledge of relative lead displacement is important, because the resulting misalignment of the anodes and cathodes of the respective leads changes the stimulation pattern, thereby degrading the therapy provided by the SCS. Without such knowledge, the previous stimulation pattern will likely have to be recovered through trial and error and patient feedback, making reprogramming of the electrodes especially difficult.
It is possible to image the patient's spinal column using conventional imaging modalities, such as fluoroscopy, to determine the occurrence and extent of lead migration. However, use of conventional imaging systems, which are often not readily available, is inconvenient and costly. In addition, detection of lead migration via conventional imaging cannot be performed remotely. Furthermore, it may be desirable to automatically program electrodes in response to detection of lead migration. However, the use of conventional imaging modalities to detect lead migration is not suitable for automated electrode programmability.
There, thus, remains a need for an improved method and system for determining the occurrence and extent of neurostimulation lead migration in a patient.